A medical condition of pain is a complicated physiological process that involves several kinds of sensory mechanisms and neural mechanisms. Pain can be defined as an unpleasant sensory or emotional experience accompanied by a real or potential tissue damage, or can be described as such a damage.
Pathophysiologically, pains can be classified into inflammatory pains and neuropathic pains.
The inflammatory pain is a nociceptive pain via nociceptor, which can be considered as a pain caused by an inflammatory mediator released to a site of tissue damage. The mechanism of an inflammatory pain is considered as follows. When a tissue is damaged and inflammation is caused, a pain producing substance such as bradykinin, ATP, or proton, and/or prostaglandin, serotonin, histamine, or an inflammatory cytokine are released, and a spontaneous pain develops incessantly. Further, hyperalgesia occurs due to hypersensitization of a nociceptor. Still further, as a mechanism of hypersensitization of a nociceptor, the phosphorylation of an ion channel, particularly TRPV1channel known as a capsaicin receptor, is reported.
On the other hand, a neuropathic pain can be defined as a pain that is caused by a lesion or a disease of a somatosensory nervous system (International Association for the Study of Pain in 2011). In Japan, it is estimated that there are several millions of neuropathic pain patients. The pain is a pain that does not involve excitation of a nociceptor, and that involves plastic changes of peripheral nerves or central nerves. As a mechanism of development of a neuropathic pain, the following have been reported: the ectopic firing of peripheral nerves; the neuroanatomical reconstruction in peripheral nerves and dorsal horns of the spinal cord; the control of descending inhibition systems; and the activation of glial cells in the spinal cord.
Several documents including Patent Document 1 disclose methods and compositions for treating pains. Further, Patent Document 2 discloses a pharmaceutical composition for suppressing abnormal splicing that occurs when kinase is over-induced.